Polyesters, especially poly(ethylene terephthalate) (PET) are versatile polymers that enjoy wide applicability as fibers, films, and three-dimensional structures. A particularly important application for PET is for containers, especially for food and beverages. This application has seen enormous growth over the last 20 years, and continues to enjoy increasing popularity. Despite this growth, PET has some fundamental limitations that restrict its application in these markets. One such limitation is its tendency to generate acetaldehyde (AA) when it is melt processed. Because AA is a small molecule, AA generated during melt processing can migrate through the PET. When PET is processed into a container, AA will migrate over time to the interior of the container. Although AA is a naturally occurring flavorant in a number of beverages and food products, in many instances the taste imparted by AA is considered undesirable. For instance, AA will impart a fruity flavor to water, which detracts from the clean taste preferred for this product.
A second limitation in the use of PET for packaging applications is its tendency to become more yellow with increased or more severe processing. This tendency to become more yellow has been associated with the presence of various aldehydes present in the polyester. One such aldehyde is the aforementioned acetaldehyde. A second aldehyde known to promote yellowing in polyesters is 4-carboxybenzaldehyde (4-CBA). 4-CBA is an impurity formed during terephthalic acid manufacture, and while methods exist to decrease the 4-CBA level of terephthalic acid to 25-100 ppm, reducing the 4-CBA content of terephthalic acid below these levels is difficult to achieve. Even at these low concentrations, 4-CBA can adversely affect the color of PET, and is thought to be a contributor to the increased yellowing of PET during recycling or after melt blending with polyamides.
PET is traditionally produced by the transesterification or esterification of a terephthalate precursor (either dimethyl terephthalate or terephthalic acid, respectively) and ethylene glycol, followed by melt polycondensation. If the end use application for the melt-polymerized PET is for food packaging, the PET is then subject to an additional operation known as solid-state polymerization (SSP), where the molecular weight is increased and the AA generated during melt polymerization is removed. A widely used method to convert the SSP PET into containers consists of drying and remelting the PET, injection molding the molten polymer into a container precursor (preform), and subsequently stretch blow-molding the preform into the final container shape.
Historically, the impact of AA on product taste has been minimized by use of low-activity polymerization catalysts to minimize regeneration of AA during injection molding, use of extended solid-state polymerization times to insure complete removal of AA prior to injection molding, and use of low-shear screws and balanced hot-runner systems to minimize AA regeneration during injection molding. Typical preform AA levels for PET preforms produced using these methods are 6-8 ug/g (ppm), which is acceptable for many applications where the taste threshold for AA is sufficiently high, or where the useful life of the container is sufficiently short. For other applications, where the desired shelf-life of the container is longer, the product is more sensitive to off-taste from AA, or the prevailing environmental conditions are warmer, it is not possible to keep the AA level below the taste threshold even by employing these methods. For example, in water the taste threshold is considered to be less than about 40 ug/L (ppb), and often a shelf-life of up to two years is desired. For a PET bottle that contains 600 ml of beverage, a preform AA content of 8 ppm can result in a beverage AA level greater than 40 ppb in as little as one month.
However, even when acceptable AA levels can be achieved using the above-described methods, achieving those AA levels comes at a significant cost. That cost includes the need to carry out a solid-state polymerization step after the melt polymerization of PET, the need for specially designed injection molding equipment, and the need for low-activity polymerization catalysts. In addition, because AA is regenerated during the injection molding process, and the amount generated is critically dependent on the injection molding process conditions, preform manufacturers must continually monitor AA content during container production.
In addition to the afore-mentioned process-related methods, other methods to minimize AA content of polyesters include modification of the polymer itself through the use of lower intrinsic viscosity (IV) resins or the use of lower melting resins. However, lower IV resins produce containers that are less resistant to environmental factors such as stress crack failure. Lower melting resins are achieved by increasing the copolymer content the PET resin, but increasing the copolymer content also increases the natural stretch ratio of the polymer, which translates into decreased productivity in injection molding and blow molding.
Methods to reduce the impact of yellowing from 4-CBA include the aforementioned purification of the terephthalic acid feedstock. Other methods include the addition of toners (especially cobalt salts, or blue and red dyes) to mask the yellowness. However, these approaches also have inherent costs, and do not completely address the issue of increasing yellow discoloration in polyesters with increasing or more severe processing, especially for recycled PET.
Another approach to minimize the AA content of polyesters has been to incorporate additives into the polyester that will selectively react with, or scavenge, the acetaldehyde that is present. Thus Igarashi (U.S. Pat. No. 4,837,115) discloses the use of amine-group terminated polyamides and amine-group containing small molecules as AA scavengers. Igarashi teaches that the amine groups are effective because they can react with AA to form imines, where the amine nitrogen forms a double bond with the AA moiety. Igarashi teaches that essentially any amine is effective. Mills (U.S. Pat. Nos. 5,258,233; 5,650,469; and 5,340,884) and Long (U.S. Pat. No. 5,266,416) disclose the use of various polyamides as AA scavengers, especially low molecular weight polyamides. Turner and Nicely (WO 97/28218) disclose the use of polyesteramides. These polyamides and polyesteramides are believed to react with AA in the manner described by Igarashi. Rule et. al. (U.S. Pat. No. 6,274,212) discloses the use of heteroatom-containing organic additives that can react with acetaldehyde to form unbridged 5- or 6-member rings, with anthranilamide being a preferred organic additive.
While these AA scavengers are effective at reducing the AA content of polyesters, they suffer from their own drawbacks. For example, relatively high loadings of polyamides or polyesteramides are needed to effect significant AA reductions, and very significant yellowing of PET can occur on incorporation of these amine-containing additives. The use of anthranilamide also results in some degree of discoloration of PET. This color formation inherently restricts the use of these additives to packaging where the PET can be tinted to mask the color. However, most PET packages in use today are clear and uncolored.
Another drawback of these approaches for controlling the AA content of PET is related to their mechanism of action in that they all depend on incorporation of an additive that reacts stoichiometrically with acetaldehyde. Consequently, the amount of AA that can be sequestered in a polyester by these additives is inherently limited to the amount of additive incorporated. Moreover, because the reaction between these additives and AA is thermodynamically reversible, the amount of additive incorporated must be substantially greater than the amount of AA to be sequestered. This limitation is especially important if relatively large amounts of AA need to be scavenged from the polyester, such as in polyesters that have been subjected to very severe processing, or in polyesters that have not had their melt-phase AA content reduced via solid-state polymerization.
A final drawback of the additives disclosed in the above references is that, to a greater or lesser degree, they all are extractable, and therefore can themselves affect the taste of food or beverages packaged in containers made from polyesters incorporating these additives.
A different method of decreasing the AA content of polyesters is disclosed by Rule (U.S. Pat. No. 6,569,479) wherein acetaldehyde present in melt-processed PET is oxidized to acetic acid by the action of an active oxidation catalyst and molecular oxygen. While this method is catalytic, and is therefore capable of removing greater than stoichiometric amounts of acetaldehyde, it suffers from the drawback that the active oxidation catalysts useful for this invention are relatively unselective and are themselves active for generating acetaldehyde under melt-processing conditions, thus limiting their effectiveness. Thus, the greatest amount of decrease in the beverage AA content disclosed by this invention is only 32%.